Exhaust diameter: 3" vs 2.5"

[M&M]

Hi guys, what you finding works best on these cars for the mid-pipes? Does the 3" make more power than the 2.5"?

[askiles]

Who makes 3" midpipes, or any other exhaust part for the M3?

-Andy

[M&M]

Originally Posted by askiles

Who makes 3" midpipes, or any other exhaust part for the M3?

-Andy

Milltek?

[J08M3]

Originally Posted by PencilGeek

Going much larger will change the back pressure, and probably have an adverse effect on performance. Larger diameter is often times worse on a NA motor.

The whole idea of any aftermarket exhaust system is to lower back pressure. Back pressure is simply nothing more than a restriction, you want to reduce it.

[maaicher]

Originally Posted by J08M3

The whole idea of any aftermarket exhaust system is to lower back pressure. Back pressure is simply nothing more than a restriction, you want to reduce it.

Yes, but there is a certain point on a NA motor that to much of a decrease in back pressure will lose power.

[askiles]

Originally Posted by J08M3

The whole idea of any aftermarket exhaust system is to lower back pressure. Back pressure is simply nothing more than a restriction, you want to reduce it.

Back pressure is not a restriction. You would not gain power if you had NO exhaust system on. You lose big amounts of power doing that. Your engine needs a certain amount of back pressure to work properly. The key is tuning an exhaust system where you have the right amount of backpressure, with the right amount of flow. That is the system that will make the most power.

-Andy

[powertrip]

Originally Posted by PencilGeek

Not true. I've seen many motors dyno'd on a real motor dyno during exhaust development. When you go too big, you lose power on a NA motor.

That's been my experience as well. Going larger than 3 inches on STI engines will lose power...period.

[J08M3]

It's not the backpressure, that creates horsepower. Back pressure is the same as a restriction. What creates the power from a tuned exhaust system is harnessing the effects of the pulse created by the engine. This pulse is used to help scavenge the exhaust gases from the cylinder and also during valve overlap it actually "sucks" more air/fuel into the cylinder. You lose power with no exhaust system because you don't get to take advantage of this pulse created in the engine.

Back pressure would simply hold back exhaust gases from leaving the cylinder and a fresh intake charge from entering. Unless some body can explain in a technical term why you would want to have pressure on the exhaust gases trying to leave the cylinder?

[J08M3]

What I'm saying in no way means there is a guaranteed advantage to 3" exhaust. Changing any component in the exhaust system varies the timing of the pulse. If that pulse is not timed properly you will have less power than with a system that it is timed properly.

This is the very same theory that works on the opposite side of the engine with intake runners. You have to have the proper length and size runner to take maximum advantage of that pulse.

[GhostyM]

Originally Posted by J08M3

It's not the backpressure, that creates horsepower. Back pressure is the same as a restriction. What creates the power from a tuned exhaust system is harnessing the effects of the pulse created by the engine. This pulse is used to help scavenge the exhaust gases from the cylinder and also during valve overlap it actually "sucks" more air/fuel into the cylinder. You lose power with no exhaust system because you don't get to take advantage of this pulse created in the engine.

Back pressure would simply hold back exhaust gases from leaving the cylinder and a fresh intake charge from entering. Unless some body can explain in a technical term why you would want to have pressure on the exhaust gases trying to leave the cylinder?

+1.

Everyone...

[GTM]

Two 2.5in system can easily support upto around 500hp+ crank before the diameter holds engine power.

Unless you are looking more like near 600hp, 3in piping will not provide enough advantages over weight/clearance issue that 3in piping will have.